Final answer:
The acidic equilibrium equation for the HPO4^2- ion is its reaction with water, forming H2PO4^- and OH-. HPO4^2- acts as a Brønsted-Lowry base, accepting a proton to become H2PO4^- in a reversible reaction.
Step-by-step explanation:
The acidic equilibrium equation for the HPO4^2- ion involves its reaction with water to form H2PO4^- and OH-. This equation is represented as follows:
HPO4^2- (aq) + H2O (l) → H2PO4^- (aq) + OH- (aq)
In this equilibrium, the HPO4^2- ion acts as a Brønsted-Lowry base, accepting a proton (H+) from the water to form the dihydrogen phosphate ion, H2PO4^-, and the hydroxide ion, OH-. This reaction is reversible and can proceed in both the forward (as written) and reverse directions depending on the pH of the solution and other conditions. Thus, in answer to the question, the acidic equilibrium equation starting with the HPO4^2- ion is the conversion of HPO4^2- to H2PO4^- and not any of the other options provided.